Release and dispersal of pollen from dwarf mistletoe on jack pine in Manitoba in relation to microclimate

The release and dispersal of pollen of Arceuthobiumamericanum Nutt. ex Engelm. parasitizing Pinusbanksiana Lamb. were examined in relation to microclimate in southern Manitoba. Time-lapse photography revealed that once open, the long-lived staminate flowers remain open. The anthers, however, open in response to rising temperatures and falling relative humidities and close under the reverse conditions. Small Diptera (Sciaridae) appeared on the photographs on 25 occasions during the wet spring of 1986. Nectar was scarce on pistillate flowers, but accumulated on the central cushions of staminate flowers when relative humidities were high. In the dry spring of 1987, nectar was rarely seen on the staminate flowers and one small dipteran was photographed just once. Large beads of concentrated nectar (50–65% sugar), however, formed on the stigmas in 1987. Large Diptera were rarely seen in 1986, but were numerous in 1987. Visits to staminate flowers were more frequent than to pistillate (2.7:1.0), but individual insects spent more time on pistillate flowers than staminate (9.6:1.0 s). Pollen grains, trapped on a continuously recording volumetric spore trap, increased in number during warm periods and fluctuated when weather alternated between rainy and dry. Pollination is effected by unspecialized insect visitors and wind. Like other diclinous species, A. americanum represents a compromise between entomophily and anemophily.